Adjustable thermal switch



Jan. 29, 1957 T. s. ZASTROW 2,779,841

ADJUSTABLE THERMAL SWITCH Filed April 50, 1954 IN V EN TOR. THO/V85 S. 29 5 TRON 'f QBZZIQQW QTTORNEY United tates Patent ADJUSTABLE THERMAL SWITCH Thomas S. Zastrow, St. Paul, Minn., assignor to General Mills, Inc., a corporation of Delaware Application April 30, 1954, Serial No. 426,726 9 Claims. (Cl. 200138) This application relates to thermal switches and more particularly to an improved adjustment means for such a switch.

It is customary in the thermal switch art to provide some means of adjustment to establish the desired operating temperature for the switch. Two types of adjustment are sometimes provided. One type involves an internal or so-called factory adjustment which may be set when the switch is initially assembled, in order that the switch operation may be standardized in spite of possible variations due to manufacturing tolerances, differences in mounting, and the like. The other type of adjustment which is often provided is an external or manual adjustment designed for use by an operator to set the desired condition of operation for the device with which the switch is used.

The present invention is concerned primarily with an adjustment means of the first type, either alone or in combination with an adjustment means of the second type just described.

It is one object of the invention to provide an improved adjustment means for a thermal switch.

Another object is the provision of an adjustment mechanism which is extremely simple and economical in construction and reliable in operation.

Other objects and advantages of the invention will be apparent from the following specification in which certain preferred embodiments are described with particular reference to the accompanying drawings. In these drawlngs,

Figure l is a partial sectional view of a thermal switch according to the invention mounted on an associated appliance, with certain portions broken away in the interests of brevity and clarity;

Fig. 2 is a partial side view of the adjusting portion of the switch of Fig. 1, showing the manner in which the temperature of operation of the switch may be varied according to the present invention;

Fig. 3 is a partial view of the adjusting mechanism of Fig. 2 taken on the line 3-3 of that figure; and

Fig. 4 is a partial schematic view similar to Fig. l of another embodiment of the invention in which an adjusting means similar to that of Fig. l is incorporated in a switch having external manually adjustable means for setting its operating temperature by the user.

With reference to Fig. l, the thermal switch indicated generally at is shown mounted on the inverted base 12 of a coffee pot or other heating vessel 14. Heat for the vessel 14 is supplied by a heating unit held in position on the bottom 12 by a suitable bracket or flange 16. The heating element can be of any known construction and is illustrated as a series of windings 18 or" resistance wire carried on an insulating core 29 and spaced by suitable outer insulators 22 from the metallic portions of the container and flange. The heating element 18 has leads 24 and 36 by which the current is supplied to it. Lead 24, for example, may be connected directly to one terminal 26 of a suitable current supply line. The other Patented Jan. 253, i957 terminal 28 of the supply line is in turn connected by lead 30 to one terminal 32 of the thermal switch 1%. The remaining terminal 34 of the switch is then connected to the remaining heating element lead 36.

The switch it) includes a suitable heat-responsive actuating member indicated at 38. Various types of heat-responsive members might be used to actuate a switch according to the invention, such as an expanding and contracting rod, the expansion and contraction of the container itself, or the diiferential expansion of two members of diiferent thermal expansion. By way of example, however, the present invention has been illustrated with a bimetallic actuator 38 having a high expansion side immediately adjacent the bottom 12 of the container and a low expansion side facing away from the container. Thus as the bimetallic member 38 increases in temperature as the container is heated, its free end will tend to move upwardly away from the bottom 12.

The thermal switch also includes a first contact arm 49 having one portion 42 suitably anchored at one end to the stationary body portion or supporting bracket of the switch. This first contact arm 40 carries a first contact 44 designed to cooperate with a second contact 46 carried by second contact arm 43. The second contact arm also has an end portion 50 designed for connection to the stationary support. The portions 42 and 50 of the contact arms, as well as the fixed end 52 of the bimetallic actuator 33, are clamped in assembled position, separated by suitable insulators 54 to prevent electrical contact between the parts. A main switch supporting bracket 56 is designed to carry the assembled parts and retain them in their proper relationship by means of one or more rivets or bolts 58 passing through the complete stack of parts as illustrated. One or more additional bolts 60 may also be provided to clamp the switch assembly to the bottom of a container or other device, the temperature of which is desired for control of the switch. In this case the bolt 69 has a head 62 within the container 1 2 and spaced therefrom by a sealing gasket 64. This bolt holds the assembly against the bottom of the pot with the bimetallic actuator 38 in direct engagement with said bottom. Insulating sleeves 55 protect the switch arms from contact with the bolts. The pot bottom may be indented as shown at 66 wherever necessary to accommodate the heads of the assembly rivets 58 and thus insure close contact between the bimetal 38 and the bottom 12.

According to the present invention, the first contact arm 4t is provided with an integral actuating flange 68 designed for engagement by the free end 70 of bimetal 38 to actuate the switch. Normally, the contact arm 4-4 may be resiliently biased toward the contact arm 48 and toward the bimetal. Then when the bimetal end 70 warps upwardly due to increasing temperature it will engage the actuating flange 63 of the first contact arm and move that arm and its associated contact away from the second contact 46.

As shown in Fig. l, the actuating flange 68 is an integral part of the contact arm 4t) but is displaced from the plane of the arm. In this case the flange 68 is located at the free end 72 of the contact arm and is in the form of an extension of said arm which is doubled back beneath the end portion 72 to provide a U-shaped portion at the end .of the arm. If desired, the flange 68 could also be located at some other movable or unclarnped portion of the contact arm 40, depending on the particular type of switch assembly needed and the desired relative location of the contacts with respect to the actuating flange 68.

In order to prevent undesired electrical contact between the actuating flange 68 and the free end 76 of the bi- 76 adapted to fit within a longitudinal slot 7 8 at the inner end of the return-bent actuating flange 68. The ends 30 of slot '78 may be slightly narrower than the body portion 76 (as shown in Fig. 3), to retain the insulator against longitudinal movement Within the slot after the parts are assembled. The insulator also includes an enlarged head or shoulder portion 32 adapted to engage the upper surface of flange 68. The enlarged portion 82 is thus located between the flange fi and the free end 72 of the contact aim.

The invention also contemplates the provision of adjusting means operatively connected between the actuating flange 68 and the contact arm. This adjusting means is designed to change the relative displacement of the flange 68 toward or away from the plane of the contact arm 40. In this instance, the adjusting means is illustrated as an adjustable screw or bolt 8 rotatably and axially movable in a threaded opening 86 in the main body of the contact arm. The end 83 of adjusting screw 84 directly engages the enlarged head 82 of the insulator '74,

which may be recessed as shown at 84 to further prevent lateral or longitudinal displacement of the insulator.

Because of the inherent resilience of the flange portion 68 it will have a normal position spaced a predetermined distance from the free end 72 of the contact arm. If the adjusting screw 84 is then turned by means of its head 92 to cause relative downward movement of the screw with respect to the upper contact arm, the actuating flange 68 will be displaced toward the second contact arm by the pressure against the head of insulator 74. The natural resilience of this flange 63 will oppose this displacement and thus tend to hold the insulator up against the bottom of the screw. This resilient engagement will also resist undesired changes in the adjustment of the screw 84. A lock nut may also be added to the screw to engage the first contact arm as further insurance against accidental rotation of the screw.

Fig. 2 illustrates the relative adjustment of the device of Fig. 1 from an initial heavy line position to a dotted line position in which the screw has been moved downwardly to displace the insulator and actuating flange farther away from the upper contact arm 49. When the parts are in the dotted line position of Fig. 2, the free end 70 of the bimetal actuator will engage the insulator at a lower temperature than when the parts are in the heavy line-position. Thus the first contact 44 would be moved away from the second contact to open the switch at a lower temperature in the dotted line position of the parts. Relative adjustment of the position of the screw 84 accordingly provides a rapid and convenient method of changing the temperature of operation of the switch.

The limits of this adjustment are sufficient to accom modate all normal production variations and even to give a substantial additional range of adjustment. This adjustment is accordingly achieved with a minimum number of parts, which can be formed and assembled at extremely low cost, and will provide a unit capable of long-continued use without undue wear or any necessity of replacement of the adjusting mechanism.

in Fig. 4 another embodiment of the invention is illustrated. Here the container 12 is provided with a switch 109 which is similar in most respects to the switch ltl of Fig. 1. Thus the switch has a bimetallic actuator '38 and a second or lower contact arm 43 identical to those of the previous embodiment.- The first contact arm 1G2, however, is provided with an additional opening 104 through which an adjusting or control shaft 1% may project. This adjusting shaft 1% is threaded into a nut or bushing 16% carried by the outer end. of the bracket or base member 112 of the switch. Thus member 112 corresponds to member 56 of the switch of Fig. 1, but with the addition of the oifset end portion, bushing 1438 and adjustable control shaft 106.

The lower end of the adjusting shaft 1436 carries an insulating button 114 adapted to engage the upper surface of contact arm 48. Such engagement limits the movement of the contact arm 4:; in a direction toward contact arm 102 and thus establishes the point at which the switch may be opened when bimetal 38 urges the first contact arm away from the second. The first contact arm includes an adjustment screw 34 and insulator '74 identical to those of the previous embodiment to vary the displacement of the actuating flange from the plane of contact arm 102 in the manner already described.

Rotary adjustment of the additional control shaft 106 will cause vertical movement of the shaft by virtue of its threaded engagement within the nut ill-3. Such rotary movement may be imparted to the shaft by a lever or crank arm 116 which projects outwardly of the device to a point where it is readily accessible for manual operation by the user. A knob 11% on the outer end of lever H6 is used for adjustment of this portion of the mechanism. Such adjustment changes the limiting position of contact 48 as already described and thus provides an additional means for varying the operating temperature at which the switch contacts will open and close.

Thus the embodiment of Pig. 4 includes both an internal or factory adjustment (adjusting screw 8d, insulator 74 and actuating flange 12%) and an external manual control (shaft res, lever 116 and knob 118). The first of these adjusting devices will normally be set at the factory, or when the device is subsequently checked at a service station, in order that the temperatures of operation may be coordinated regardless of manufacturing variations or other changes. The external control knob 118 will then be customarily set by the user to establish the desired operating temperature of the appliance. Because of the auxiliary internal adjustment at screw 84 a plurality of appliances can be produced in which the same position of the external control knob 118 will result in identical operating temperatures from one appliance to the other.

According to the foregoing description, certain embodiments of the invention have been described which essentially accomplish the objects set forth at the beginning of this specification. The adjusting device or" the invention is extremely simple in construction and assembly, involves a minimum of parts and achieves the desired stability in actual operation. Since minor variations and changes in the exact details of construction will be apparent to persons skilled in this field, it is intended that this invention shall cover all such changes and modifications as fall within the spirit and scope of the attached claims.

Now therefore I claim:

1. Ina thermal switch comprising a resiliently movable contact arm having a first contact thereon, a second contact adapted to be engaged and disengaged by said first contact, and thermally responsive means for establishing relative movement of said arm and first contact toward and away from the second contact in response to predetermined temperature conditions, the improvement comprising a projecting flange integrally connected to the contact arm and doubled back generally parallel to the contact arm in a plane displaced from the plane of the arm, aninsulating member carried by said projecting flange at a location adapted for engagement by the thermally responsive member to control the position of said arm and cause said relative movement, and an adjusting screw threaded into the contact arm opposite said insulating member, the end of said screw engaging the insulating member, thereby changing the relative spacing between the insulating member and contact arm and varying the temperature of operation of the switch in response to adthe contact arm, said outer free end being doubled back between the arm and the thermally responsive member.

3. A thermal switch according to claim 1 in which the doubled-back flange of the contact arm has a slot therein, said insulating member having a body portion fitting the slot and an enlarged head portion between the flange and arm and said screw engaging the enlarged head portion and urging it against said flange, thereby holding the body portion within said slot.

4. In a thermal switch comprising a resiliently movable contact arm having a first contact thereon, a second contact toward which said first contact is biased by said arm, and thermally responsive means for engaging said arm and automatically moving said arm and first contact away from the second contact in response to a predetermined temperature condition, the improvement comprising a projecting flange integrally connected to the contact arm and doubled back generally parallel to the contact arm in a plane displaced from the plane of the arm in the direction toward which said arm is biased, an insulating member carried by said projecting flange at a location adapted for engagement by the thermally responsive member to move said arm in the opposite direction and open the contacts, and an adjusting screw threaded into the contact arm opposite said insulating member, the end of said screw engaging the insulating member, thereby changing the relative spacing between the insulating member and contact arm and varying the temperature of operation of the switch in response to adjustment of the screw.

5. A thermal switch according to claim 4 in which said second contact is carried by a second contact arm resiliently biased toward said first arm, and manually adjustable stop means limiting the resiliently biased movement of said second arm toward the first arm and thereby providing a primary adjusting means for the temperature of operation of said switch, said adjusting screw constituting a secondary or factory adjustment for the operating temperature.

6. An adjustable switch contact arm sub-assembly comprising a movable switch arm having one portion adapted to be anchored in a switch assembly, an integral flange spaced along the arm from said one portion and doubled back generally parallel to the arm in a plane displaced from the plane of the arm, and adjusting means operatively connected between said doubled-back flange and said arm for changing the relative spacing between said flange and arm.

7. An adjustable switch contact arm assembly comprising a movable switch arm having one free end doubled back generally parallel to the arm and thereby forming a U-shaped portion at said end, adjusting means connected between said doubled-back end and said arm, and means responsive to movement of said adjusting means changing the relative spacing between said end and arm and thereby adjusting the operating position of said switch.

8. An adjustable switch contact arm assembly according to claim 7 in which said adjusting means includes an insulating member carried by the doubled-back end, and a bolt threaded into said arm and engaging said insulating member.

9. An adjustable switch contact arm assembly according to claim 8 in which the doubled-back end is resiliently biased toward said arm and bolt, thereby holding the insulating member in the adjusted position determined by the adjustment of said bolt.

References Cited in the file of this patent UNITED STATES PATENTS 2,008,163 Walder July 16, 1935 2,191,670 Kuhn et al Feb. 27, 1940 2,321,338 Ulanet June 8, 1943 2,480,216 Bowland Aug. 30, 1949 2,692,317 Bletz Oct. 19, 1954 

